Upon review of www.cdc.gov/growthcharts and conversations over the last five days, here's my two cents.

From talking to a small group of cat sailors a general consensus is that there are some expression of desire amongst kids to skipper their own boat, or a parent's desire to show them that they can (case of timid child) skipper a boat around the age of 10 or 11. One person questioned stated she was "forced to skipper" by her father. From year's of coaching, and involved in, youth sports this is a concern, at least in the US, where parents (esp. fathers) want to relive their childhood through there children. This is of note regarding existing sailors, but probably of no relevance to a new sailing parent and child. The general opinion was the Wave is a bit too much to handle at this age.

Reviewing the charts reveal these weights at 50 percentile (average)
Girls at age 10, 32kg
Girls at age 19, 57kg

Boys at age 10, 32kg
Boys at age 19, 69kg

Of note boys weight is still heading up rather sharply at 19 while girls start to level off (by comparison) at 16. Girls also attain a plateau in height at 15, whereas in boys this occurs closer to 17.5 years. Interesting also at 13 years old girls and boys are about the same weight 45 and 46kg.

Wouter based on capacity vs. hull length it looks as though something towards 3.90 mtr looks inviting. I can easily envision a pair of kids up to 90kg (13 yo) wanting to sail (play, not race) together.

Of course this does not preclude a 12 foot version made from a minimal amount of 4x8 sheet material. I agree with the notion of replacing the originals with a faster design and passing down the starter hulls.

Tacking ability edges out looks for a starter hull design, especially from a newbie standpoint.